An MRI is a system, wherein a portion or an entire subject is inserted into a hollow space of a magnetic field generating apparatus forming a strong magnetic field to obtain a tomographic image of the subject, thereby to draw out even characteristics of the tissue.
The hollow space of the magnetic generating apparatus must be so large that the porting or entire subject can be inserted, and for the purpose of obtaining the clear tomographic image, usually, it is necessary to form a strong stable uniform magnetic field having an accuracy of 0.22 to 2.0T or less at 1.times.10.sup.-4 .OMEGA..multidot.m, in an image pickup visual field (in a spherical space of about 150 to 250 mm radius at the center porting of the hollow space).
As a magnetic field generating source of the magnetic field generating apparatus, though those using a superconductive magnet and as paraconductive magnet are known, in recent years, those using a permanent magnet are frequently used by the reason that, the apparatus has a relatively simple configuration and the running cost is low.
As the magnetic field generating apparatus for MRI using the permanent magnets as the magnetic field generating source, a configuration, in which a pair of permanent magnet constituents opposing one another as forming a hollow space are connected by a yoke as shown in the U.S. Pat. No. 4,672,346(EP 0161782 B1) is widely adopted.
In the above-mentioned magnetic field generating apparatus, the permanent magnet constituents are usually constituted by a plural number of permanent magnet blocks by the reason of manufacturing problems and assembling workability.
In this configuration, since a magnetizing direction of the permanent magnet constituents is in the same direction as a direction of magnetic field in the hollow space, and also a transverse cross section is substantially a square, a direction of thickness of the permanent magnet constituents is the magnetizing direction, thus it is easy to assemble by laminating and integrating the plural number of cubical or rectangular parallel-piped permanent magnet blocks having the magnetizing direction in the direction of thickness.
While, a configuration, in which a plural number of permanent magnet constituents are disposed angularly, and an uniform magnetic field is formed at a center portion in the permanent magnet constituents, is known (U.S. Pat. No. 4,498,048, EP 0120927 B1 and Japanese Patent pablication No. Hei 4-10334).
In the magnetic field generating apparatus having such configuration, since respective permanent magnet constituents have, for example, a sectionally trapezoid shape, and the direction of magnetic field in the hollow space differs form the magnetizing direction of the permanent magnet constituents depending on locations where the permanent magnet constituents are arranged, in order to obtain a desired magnetizing direction, the permanent magnet blocks must be formed into a specific shape, or at the time of their magnetization, a magnetizer capable of positioning the permanent magnet constituents very precisely is required, results in a very complicated assembling work.
As the configuration for solving such problem, a configuration, in which a plurality of bar-shaped permanent magnet constituents having a triangular or substantially square and triangular transverse section are disposed angularly along the inner surface of a polygonal tubular yoke, is proposed (U.S. Pat. No. 4,695,802, EP 0170318 B1 and Japanese Patent publication No. Hei 4-75008).
As compared with the configuration, in which a pair of permanent magnet constituents are oppositely disposed in the yoke as forming the hollow space, in this configuration, though, a magnetic leakage is little and the permanent magnets can be used effectively, since the magnetizing direction of the permanent magnet constituents having a triangular transverse section is that, the permanent magnet blocks magnetized respectively in the direction of thickness are overlapped one another so as to have the different magnetizing directions, for example, to have the magnetizing directions which differ form each other by 90 degrees, and the magnetizing direction as the entire permanent magnet constituents are brought in parallel (same direction) and/or perpendicularly to the direction of the magnetic field in the hollow space, it is not always effective from a viewpoint of assembling work and is not practically used.
Furthermore, as an improved magnetic field generating apparatus of the configuration, in which the pair of the permanent magnet constituents are opposing disposed in the yoke as forming the hollow space (U.S. Pat. No. 4,672,346) and the configuration, in which the plurality of permanent magnet constituents are disposed angularly and the uniform magnetic field is formed at the center in the permanent magnet constituents (U.S. Pat. No. 4,498,048), the configuration, in which a pair of permanent magnet constituents consisting of a trapezoid transverse section and a permanent magnet constituent consisting of a scalene triangular transverse section are connected in a picture-frame shape, or the oblique side of the trapezoid permanent magnet constituent and the short side of the scalene triangular permanent magnet constituent are contacted closely along the inner surface of the polygonal tubular yoke, is proposed (Japanese Patent application Laid Open No. Sho 62-177903).
A configuration, in which permanent magnet pieces for correcting non-uniform magnetic field are disposed on both axial ends of the trapezoid permanent magnet constituent and the scalene triangular permanent magnet constituent, is also proposed (Japanese Patent Application Laid Open No. Sho 63-43304).
However, in these configuration, the trapezoid permanent magnet constituent, scalene triangular permanent magnet constituent and the non-uniform magnetic field correcting permanent magnet pieces are all characterized by having the magnetizing direction which is perpendicular to the boundary face with the hollow space (opposite faces in the hollow space), and among the permanent magnet blocks constituting the individual permanent magnet constituent, the permanent magnetic constituent in the vicinity of the contact with the yoke is complicated to be processed, magnetized and assembled from a viewpoint of its magnetizing direction.
Also, since the permanent magnet constituents are closely disposed one another in the picture-frame shape as previously described, the permanent magnet constituents magnetically repulse one another at the close-contact portion, thus it is problematic from the safety point of view, and further, reproducibility of obtaining the desired magnetic field uniformity always at a mass production is also problematic.
It is, therefore, a primary object of the present invention to provide a magnetic field generating apparatus having a good magnetic efficiency, in which an advantage inherent to a magnetic field generating apparatus for MRI constituted by disposing a plurality of permanent magnet constituents angularly along the inner surface of a polygonal tubular yoke is adopted positively, particularly, a magnetizing direction of a plural number of permanent magnet blocks constituting the permanent magnet constituents is directed in a specific direction to facilitate the assembling work, and the desired magnetic field uniformity is always obtained with good reproducibility at a mass production.
It is another object of the present invention to provide a magnetic field generating apparatus, in which permanent magnet constituents and a yoke capable of reducing generation of eddy currents influenced due to gradient magnetic field coils, and further, magnetic polar pieces capable of reducing generation of the eddy currents and improving the magnetic field uniformity are arranged.
It is a further object of the present invention to provide a magnetic field generating apparatus, in which by devising the arrangement of permanent magnet constituents generating the magnetic field most efficiently in the hollow space, and the shape of permanent magnet constituent itself, the magnetic field uniformity is further improved.